1. Field of Invention
The present invention relates to a liquid crystal display device and electronic apparatus. Particularly, the invention relates to a liquid crystal display device having a cholesteric liquid crystal layer as a reflective layer or a transflective layer, and presenting excellent visibility with a bright display and a wide viewing angle.
2. Description of Related Art
Reflective-type liquid crystal display devices can be applied in a variety of mobile electronic apparatus because they do not have a light source, such as a backlight, and thus they consume less power. Reflective-type liquid crystal display devices take advantage of ambient light, such as natural light or illumination light, but are difficult to view under dark conditions. Liquid crystal display devices have been proposed which use ambient light under light conditions as in an ordinary reflective-type liquid crystal display device, while presenting a display via an internal light source under dark conditions. This type of liquid crystal display device employs a reflective and transmissive display method. Depending on the ambient light level, the device switches between a reflective mode and a transmissive mode. The device presents a distinct display under dark conditions while saving power. In this specification, this type of liquid crystal display device is referred to as a xe2x80x9ctransflective-type liquid crystal display device.xe2x80x9d
In one proposed transflective-type liquid crystal display device, a reflective layer, formed of a metal film of aluminum or the like and having slits (apertures), is arranged on the inner surface of a lower substrate (in this specification, one surface of the substrate facing the liquid crystal is referred to as an xe2x80x9cinner surfacexe2x80x9d, and the other surface of the substrate opposite from the inner surface is referred to as an xe2x80x9couter surfacexe2x80x9d), and the reflective layer functions as a transflective layer.
FIG. 11 illustrates a related art transflective liquid crystal display device having this type of transflective layer.
The liquid crystal display device 100 includes liquid crystal cells including a liquid crystal 103 between a pair of transparent substrates 101 and 102. A reflective layer 104 and an insulator 106 are laminated on the lower substrate 101. A lower electrode 108, which is an electrically conductive transparent layer fabricated of indium tin oxide (hereinafter referred to ITO), is formed on the insulator 106. An alignment layer 107 is formed to cover the lower electrode 108. On the other hand, a color filter 109 having color layers of R (red), G (green), and B (blue) is formed on the upper substrate 102. A planarization layer 111 is laminated on the color filter 109. Upper electrodes 112, fabricated of electrically conductive transparent film, such as of ITO, is formed on the planarization layer 111. An alignment layer 113 is then deposited to cover the upper electrodes 112.
The reflective layer 104 is fabricated of a metal, having a high light reflectance, such as aluminum, and has a slit 110 to transmit light for each pixel. Through the slit 110, the reflective layer 104 can function as a transflective layer (hereinafter, the reflective layer 104 is referred to as a xe2x80x9ctransflective layerxe2x80x9d). Arranged on the outer surface of the upper substrate 102 are a forward diffuser 118, a retardation film 119, and an upper polarizer 114 in that order from the upper substrate 102. Arranged on the outer surface of the lower substrate 101 are a xc2xc-wave plate 115, and a lower polarizer 116 in that order from the lower substrate 101. A backlight 117 (an illumination device) is arranged beneath the lower polarizer 116 below the bottom surface of the lower substrate 101.
When the liquid crystal display device 100 shown in FIG. 11 is used in a reflective mode under light conditions, external light, such as sunlight or illumination entering from above the upper substrate 102, is transmitted through the liquid crystal 103, is reflected from the surface of the transflective layer 104 on the lower substrate 101, is transmitted through the liquid crystal 103 again, and then exits toward the upper substrate 102. When the liquid crystal display device 100 is used in a transmissive mode under dark conditions, light emitted from the backlight 117 arranged below the lower substrate 101 is passed through the slit 110 of the transflective layer 104, is transmitted through the liquid crystal 103, and then exits toward the upper substrate 102. These list rays contribute to image displaying in each mode.
In the transflective-type liquid crystal display device and the reflective-type liquid crystal display device, a metal film having a high light reflectance, such as aluminum or silver, has been used in the related art for the reflective layer or the transflective layer. A dielectric mirror can be formed of a laminate of dielectric thin films having different refractive indices, a reflective cholesteric plate formed of a cholesteric liquid crystal, or a reflective hologram plate using a hologram element is used for the reflective layer in the reflective-type liquid crystal display device. These reflective plates not only reflect light, but also have other functions.
In particular, the cholesteric liquid crystal exhibits a liquid crystal phase above a certain temperature (liquid crystal transition temperature), in which liquid crystal molecules take a cyclical helical structure configuration with a constant pitch. This structure has the property that the cholesteric liquid crystal selectively reflects light having a wavelength coinciding with the helical pitch thereof while transmitting light having other wavelengths. The helical pitch is controlled by the intensity of ultraviolet light or temperature at the curing of the liquid crystal. The color of reflected light is localized, and the cholesteric liquid crystal is thus used as a reflective-type color filter.
If a plurality of cholesteric liquid crystal layers reflecting light rays of different colors are laminated, the cholesteric liquid crystal layers function as a reflective plate that reflects white light.
The reflective plate using the cholesteric liquid crystal has the characteristic functions as described above. In comparison with the widely used metals in the related art, the cholesteric liquid crystal accomplishes a very bright and pure-color display. The reflective plate using the cholesteric liquid crystal can be used to enhance image quality on a reflective-type or transflective-type liquid crystal display device.
When a reflective cholesteric plate is used as a reflective layer to enhance the image quality on a reflective-type liquid crystal display device, a viewing angle of the screen is narrow compared with a related art display using a metal layer. Since the reflective cholesteric plate exhibits a sharp directivity in reflected light beams, a display much brighter than a related art display is obtained when a user views the screen of the liquid crystal display device within a limited narrow angle range. When a user changes the user""s viewing position, the screen suddenly becomes darker.
In a related art transflective liquid crystal display device shown in FIG. 11, the user views the display regardless of the presence or absence of ambient light. The lightness level of the screen during the transmissive mode is significantly lower than that during the reflective mode. This is attributed to the fact that the display during the transmissive mode uses only half the light beams emitted from a backlight, that the display during the transmissive mode uses the light beams passed through the slits of the transflective layer, and that the xc2xc-wave plate and the lower polarizer are arranged on the outer surface of the lower substrate.
In the related art transflective liquid crystal display device, the display mode changes between during reflection and during transmission of light. During transmission, approximately half the light emitted from the backlight is absorbed by the upper polarizer, and approximately remaining half of the emitted light is used to provide display. Specifically, linearly polarized light incident from the upper substrate is fully used for light display during the reflective mode. During the transmissive mode, light traveling from the lower surface of the liquid crystal layer to the upper substrate must be substantially circularly polarized to present a display of the same lightness level as that presented during the reflective mode. Since approximately half the circularly polarized light is absorbed by the upper polarizer when the light exits from the upper substrate. As a result, approximately only half the light incident on the liquid crystal layer contributes to image displaying. In the basic principle, the related art transflective liquid crystal display device inherently provides a dark display during the transmissive mode.
During the transmissive mode, the display is presented making use of light passed through the slits. The area of the slit to the entire area of the transflective layer (namely, an aperture ratio) determines the lightness level of the display. If the aperture ratio is increased, the display becomes bright during the transmissive mode. With a high aperture ratio, however, the non-aperture area of the transflective layer decreases, darkening the display during the reflective mode. To assure the brightness of the display during the reflective mode, the aperture ratio of the slits must not be increased above a certain limit. The brightening of the display during the transmissive mode is thus subject to a limitation.
The basic principle of the transflective-type liquid crystal display device requires the use of the xc2xc-wave plate on the outer surface of the lower substrate. The reason why the liquid crystal display device lacks brightness because of the xc2xc-wave plate during the transmissive mode is discussed. In the discussion that follows, a dark display is presented with a non-selection voltage applied state while a light display is presented with a selection voltage applied state.
A dark display during the reflective mode in the transflective liquid crystal display device 100 illustrated in FIG. 11 is explained. When the transmission axis of the upper polarizer 114 is parallel with the plane of the page, the light incident on the outer surface of the upper substrate 102 from outside becomes linearly polarized light having the polarization axis parallel with the plane of the page when being transmitted through the upper polarizer 114 over the upper substrate 102, and then becomes generally circularly polarized light through birefringence of the liquid crystal 103 when being transmitted through the liquid crystal 103. The light becomes reverse circularly polarized light when being reflected from the surface of the transflective layer 104 on the lower substrate 101. When being transmitted through the liquid crystal 103 again, the light becomes linearly polarized light having the polarization axis perpendicular to the plane of the page, and then reaches the upper substrate 102. Since the upper polarizer 114 above the upper substrate 102 has the transmission axis thereof parallel with the plane of the page, the light reflected from the transflective layer 104 is absorbed by the upper polarizer 114, thereby failing to return to the outside of the liquid crystal display device 100 (to a viewer). The liquid crystal display device 100 thus presents a dark display.
When a light display is presented during the reflective mode, the alignment direction of the liquid crystal 103 is changed in response to the application of a voltage to the liquid crystal 103. Ambient light incident from outside the upper substrate 102 becomes linearly polarized light when being transmitted through the liquid crystal 103. The light is reflected from the transflective layer 104, and is transmitted through the upper polarizer 114 above the upper substrate 102 as linearly polarized light having the polarization axis parallel with the plane of the page, and then returns to the outside (to the viewer). The liquid crystal display device 100 thus presents a light display.
When a display is presented on the liquid crystal display device 100 during the transmissive mode, light emitted from the backlight 117 is incident on the liquid crystal cell from outside the lower substrate 101, and a portion of the light passed through the slits 110 contributes to image displaying.
To present a dark display on the liquid crystal display device 100, the light traveling from the slit 110 to the upper substrate 102 must be generally circularly polarized in the same manner as during the reflective mode as already described. Since the light emitted from the backlight 117 and passed through the slit 110 must be generally circularly polarized, the xc2xc-wave plate 115 is required to convert the linearly polarized light, after being transmitted through the lower polarizer 116, into generally circularly polarized light. The xc2xc-wave plate has the capability to convert the linearly polarized light into generally circularly polarized light at a certain wavelength.
A portion of light emitted from the backlight 117 but not passed through the slit 110 is discussed. When the transmission axis of the lower polarizer 116 is perpendicular to the plane of the page, the light emitted from the backlight 117 becomes linearly polarized light having the polarization direction perpendicular to the plane of the page when being transmitted through the lower polarizer 116. The linearly polarized light then becomes generally circularly polarized light when being transmitted through the xc2xc-wave plate 115, and reaches the transflective layer 104. When the light is then reflected from the bottom surface of the transflective layer 104, the light becomes reverse circularly polarized light. When being transmitted through the xc2xc-wave plate 115 again, the light becomes linearly polarized light having the polarization axis thereof parallel with the plane of the page. The linearly polarized light is then absorbed by the lower polarizer 116 having the transmission axis thereof perpendicular to the plane of the page. In other words, out of the light emitted from the backlight 117, the portion of the light not passed through the slit 110 is reflected from the bottom surface of the transflective layer 104, and is mostly all absorbed by the lower polarizer 116 below the lower substrate 101.
Almost all of the light that is not passed through the slit 110 and reflected from the transflective layer 104 during the transmissive mode is absorbed by the lower polarizer 116 below the lower substrate 101 in the transflective-type liquid crystal display device 100. This means that only a fraction of the light emitted from the backlight 117 contributes to image displaying. If the light emitted from the backlight 117 is transmitted through the lower polarizer 116 without being absorbed by the lower polarizer 116, and returns to the backlight 117, the light just emitted from the backlight 117 and the returning light effectively heighten luminance of the backlight 117. The lightness level during the transmissive mode is heightened. If the light that is not passed through the slit 110 and then reflected from the transflective layer 104 is reused, the lightness level during the transmissive mode is heightened. The related art cannot achieve the reuse of the reflected light.
The present invention has been developed to address or resolve this problem, and the present invention provides a reflective-type or a transflective-type liquid crystal display device employing a cholesteric liquid crystal layer as the reflective layer thereof and presenting an excellent visibility with a wide viewing angle and a bright display.
The present invention also provides electronic apparatus including the liquid crystal display device having excellent visibility.
To address or achieve the above advantages, a liquid crystal display device having a liquid crystal cell including an upper substrate, a lower substrate facing the upper substrate, and a liquid crystal layer encapsulated between the upper substrate and the lower substrate, includes a reflective layer formed of a cholesteric liquid crystal layer arranged on the inner side of the lower substrate, and having a plurality of unflat portions, to reflect at least a portion of elliptically polarized light having a predetermined rotation direction, and an upper-substrate side elliptically-polarized-light input device to cause elliptically polarized light to enter the liquid crystal layer from the upper substrate. The liquid crystal layer reverses the component of the elliptically polarized light that is incident during one of a selection electric field applied state and a non-selection electric field applied state, and does not change the component of the elliptically polarized light during the other of the selection electric field applied state and the non-selection electric field applied state.
The liquid crystal display device of the present invention employs the cholesteric liquid crystal layer as the reflective layer imparting a sharp directivity to the reflected light. However, since the cholesteric liquid crystal layer has the plurality of unflat portions, the elliptically polarized light having the predetermined rotation direction is scattered when being reflected by the reflective layer and output therefrom. In other words, when the cholesteric liquid crystal layer has a plurality of unflat portions, the helical structure of the liquid crystal molecules forming the cholesteric liquid crystal layer is inclined in a wide angle range. The light incident on the reflective layer is scattered when being reflected from the cholesteric liquid crystal layer, thereby being output as a light beam having a wide angle.
In this way, the liquid crystal display device of this invention provides an intensity distribution of the reflected light milder than that of the related art liquid crystal display device with the cholesteric liquid crystal layer having no unflat portions. The liquid crystal display device providing excellent legibility with a wide viewing angle and bright display results.
In the liquid crystal display device, the reflective layer preferably converges the reflected light in a predetermined angle range.
The direction in which the viewer views the display screen (namely, a direction facing the front of the viewer) is approximately normal to the plane of the substrate. In the above liquid crystal display device, the reflective layer converges the reflected light within a predetermined angle range. By converging the light reflected from the reflective layer in a direction approximately normal to the substrate, the amount of light reflected in a direction other than the direction in which the viewer views is reduced. The reflected light is thus effectively used as light contributing to an increase in the lightness level when the viewer views the display screen. An even brighter display results.
When the direction of the light reflected from the reflective layer agrees with the direction of light which is specularly reflected from the display screen of the liquid crystal display device, the light exiting from the liquid crystal display device superimpose the external light reflected from the surface of the liquid crystal display device, making the display less recognized. In the above-referenced liquid crystal display device, the reflective layer causes the direction of the reflected light to be different from the specular reflection direction on the surface of the liquid crystal display device. The liquid crystal display device thus presents a very recognizable display.
In the liquid crystal display device, the unflat portion of the cholesteric liquid crystal layer is preferably formed of a curved surface.
In this liquid crystal display device, light is effectively scattered when the light is reflected from the reflective layer. The intensity distribution of the reflected light becomes mild. As a result, the liquid crystal display device presents an excellent visibility with a wide viewing angle and bright display.
The liquid crystal display device preferably includes an underlayer having a plurality of unflat portions, beneath the cholesteric liquid crystal layer, on the side of the lower substrate.
In this liquid crystal display device, the cholesteric liquid crystal layer having the plurality of unflat portions is easily fabricated. The liquid crystal display device presenting an excellent visibility with a wide viewing angle and a bright display results.
In the liquid crystal display device, the underlayer is preferably fabricated of a resin.
In this liquid crystal display device, the configuration of the unflat portions of the underlayer is easily controlled. The unflat portion having an arbitrary configuration is easily formed. A plurality of unflat portions forming the cholesteric liquid crystal layer is easily formed. With the unflat portion appropriate for a wide viewing angle, a liquid crystal display device having an even better visibility is provided.
The liquid crystal display device may further include a color filter layer having a plurality of color layers containing pigments of different colors over the reflective layer on the side of the upper substrate.
The liquid crystal display device thus presents a color display.
Preferably, the liquid crystal display device further includes an illumination device to cause light to enter the liquid crystal cell through the lower substrate, and a lower-substrate side elliptically-polarized-light input device to cause elliptically polarized light to enter the liquid crystal layer from the lower substrate.
The liquid crystal display device of the present invention is embodied as a transflective-type liquid crystal display device with the lightness level equalized between during the transmissive display mode and the reflective display mode. To this end, some method is required to cause light to enter the liquid crystal cell through the lower substrate, and to cause elliptically polarized light to enter from the lower substrate toward the liquid crystal layer. Any method is acceptable. For example, a so-called backlight may be arranged as an illumination device to cause the light from the lower substrate to enter the liquid crystal cell. The light is thus easily incident from the lower substrate.
In the liquid crystal display device, the underlayer having the plurality of unflat portions presents a bright display in a wide viewing angle during the reflective mode for the reason described above. During the transmissive mode, the liquid crystal display device presents a bright display for the reason to be discussed below. In comparison with the related art transflective-type liquid crystal display device, the transflective-type liquid crystal display device of this invention presents an excellently visible display both during the reflective mode and during the transmissive mode.
The cholesteric liquid crystal has the so-called selective reflectivity in which the cholesteric liquid crystal selectively reflects circularly polarized light having the wavelength thereof equal to the helical pitch of the liquid crystal molecules and having the rotation in the same direction as that of the helical structure. In other words, circularly polarized light having a wavelength that is not equal to the helical pitch of the liquid crystal molecules and circularly polarized light having a wavelength equal to the helical pitch, but having the rotation in a direction reverse to that of the helical structure are transmitted through the cholesteric liquid crystal. The cholesteric liquid crystal layer here does not fully transmit the circularly polarized light having a wavelength equal to the helical pitch of the liquid crystal molecules and having a rotation in the same direction as that of the helical structure of the liquid crystal molecules. Specifically, the cholesteric liquid crystal layer reflects a portion of the circularly polarized light while transmitting the remaining portion of the circularly polarized light. In this way, the cholesteric liquid crystal layer functions as a transflective layer.
The inventors of the present invention have found that using the reflective layer of the cholesteric liquid crystal currently proposed in the reflective-type liquid crystal display device provides the following feature. When the light incident on the liquid crystal cell is elliptically polarized, and when the liquid crystal mode is set so that the component of the elliptically polarized light is reversed during one of the selection electric field applied period and the non-selection electric field applied period for the liquid crystal layer, the lightness level in the display mode is equalized between reflection and transmission, and the lightness level is not lowered in the basic principle during the transmissive mode. The inventors have also found that the light reflected from the lower substrate due to the selective reflection of the cholesteric liquid crystal is reused with the related art construction outside the lower substrate remaining unchanged. Based on these facts, the present invention has been proposed.